The present invention relates to method for reducing complex frequency down-conversion impairments.
There are several frequency down-conversion techniques for passband signals. The most common are:
1. Direct conversion: The passband signal is down-converted directly to baseband. This scheme minimizes the number of associated components, but usually suffers from noise generated by several sources (DC offsets, power supply noise, 1/f noise, etc.).
2. Single/Dual conversion: The received signal is down-converted to an intermediate frequency (IF) from which it is demodulated/sampled and demodulated. This conversion has a major downfall known as the image frequency interference; signals residing at the frequencies fLOxc2x1fIF both downconvert to the same IF frequency. One solution to this problem is to filter the image signal before down-converting it; another solution is complex down-conversion.
3. Mixed real and complex down-conversion: This conversion involves both previous conversions, where the first one or two conversions are real and the last one is complex.
Complex conversion uses two local oscillators (shifted by 90 degrees) and two mixers to form two orthogonal output signals. One is referred to as xe2x80x9cRealxe2x80x9d and the other as xe2x80x9cImaginaryxe2x80x9d. This complex down-conversion has no image frequency problem as long as the two signal paths are symmetric. Implementing complex conversion using analog components (phase shifter and mixer) causes phase, gain and delay mismatches. The first two delays are directly addressed by the present invention. The delay mismatch can be regarded as phase mismatch as long as the signal bandwidth is relatively small with respect to its center frequency.
The present invention provides a signal processing method for substantially reducing gain and phase mismatches, which occurs in analog complex conversion. The gain and phase imbalance in complex conversion result from limited image rejection in the frequency conversion. The present invention provides a method to estimate the phase and gain mismatches by means of signal processing and then compensating for the estimated impairments. Using the method of the present invention, image rejection in the complex conversion is enhanced.